The process of electroerosive traveling-wire or wire-cut machining generally makes use of a continuous electrode wire composed of, say, brass or copper, and having a thickness ranging between 0.05 and 0.5 mm. The continuous electrode wire is axially advanced along a given continuous guide path from a supply means, e.g. a wire-storage drum to a takeup means, e.g. a wire-collection drum through a workpiece disposed in a predetermined cutting zone. A pair of wire-guide members are disposed across the workpiece to define in the cutting zone a straight line path therebetween designed for the electrode wire precisely to travel axially traversing the workpiece.
The cutting gap is flushed with a cutting fluid and electrically energized with a high-density electric current which is passed between the electrode wire and the workpiece to electroerosively remove stock from the workpiece. Advantageously, the cutting fluid is a distilled water liquid or any dielectric medium and the electroerosive machining current is in the form of a succession of electrical pulses which results in a repetition of time-spaced, discrete, localized electrical discharges across the cutting gap. Each individual electrical discharge striking on a random localized area on the workpiece surface acts to impulsively melt and vaporize stock which is dislodged impulsively from that area under a high pressure accompanying the electrical discharge. With machining pulses successively applied across the gap, such electrical discharges are effected repetitively but on varying localized areas, thereby cumulatively removing stock from the workpiece.
As the electroerosive stock removal proceeds, the workpiece is displaced relative to the aforementioned straight line path transversely thereto. This allows the electrode wire which is traveling axially to advance transversely to the workpiece and consequently a cutting slot to be formed behind the advancing electrode wire. The continuous relative displacement along a prescribed path results in the formation of a desired contour corresponding thereto and defined by this cutting slot in the workpiece. To assure a cutting accuracy, it is vitally important that the electrode wire be held axially to travel precisely in alignment with the straight line path and hold its linearity between the wire-guide members across the workpiece.
Heretofore it has been commonly believed that this requirement of linearity of the traveling electrode wire would be inherently met by the provision of traction means, i.e. wire-braking means disposed on the wire-supply side in conjunction with wire-drive means disposed on the wire-takeup side, needed to hold the traveling electrode wire tightly stretched across the wire-guided members of the cutting zone. It has now been recognized, however, that in spite of the effort to tightly stretch the electrode wire with the traction means, the required cutting accuracy is not necessarily obtainable and that a cutting inaccuracy is created in practice by reason of the fact that considerable pressure develops accompanying the electroerosive discharges and tends to deflect or force back the electrode wire traveling axially in the cutting zone. As a result, the traveling electrode wire tends to lag behind the preset straight line path defined by the wire-guide members. This means that at any instant the actual position of the axis of the electrode wire deviates from the preset position relative to the workpiece and the wire axis actually may not precisely follow a prescribed cutting path in the workpiece. Furthermore, it has been found that the magntiude of the electrical discharge pressure and hence the extent of the wire deflection may vary actually from time to time. Thus, when the prescribed cutting path not a single rectilinear path but is as customarily is, curved and/or cornered, the direction and extent of deviation would vary from point to point and eventually the actual contour of cutting would not precisely reproduce the desired contour of cut. It has now also been found that the cutting inaccuracy created in the manner described is even further accentuated when the machining fluid is delivered or forced to flow uncontrolledly or in a customary manner into the cutting gap.